Vehicle seats have actuators to adjust certain attributes such as the height or position of the seat. When the power is turned off, this type of actuator can keep the seat in its position without the need of other devices. This type of actuator has a motor and a gearbox, and requires the gearbox to have a large load carrying capacity and high transmission efficiency. The gearbox has a two stage gear train. The first stage includes a worm and worm wheel. The second stage is a N-N type planetary or epicyclic gear train. FIG. 1 illustrates an N-N type gear train used in a known seat actuator. The N-N type gear train includes an internally toothed ring 1 having an output gear, a shaft 2, an internally toothed ring 3, a planetary gear 4, a bearing 5, an eccentric wheel 6 (inserted in the worm wheel). The internally toothed ring 3 and the planetary gear 4 form an internal mesh N1 there between. The internally toothed ring 1 having the output gear and the planetary gear 4 form an internal mesh N2.
The main problem of the conventional actuator is that the N-N type planetary gear train has a low transmission efficiency and therefore the efficiency of the corresponding gear motor is also low. Another problem of the conventional adjustment device is that the gear-box has a low load carrying capacity. This is because the load is borne by only one or two pairs of gears in the N-N type planetary gear train and there are only a small number of teeth that are in mesh at any time.